1. Technical Field of the Invention
The present invention relates in general to the mobile communications field and, in particular, to a method for controlling transmission power levels in a spread spectrum or Code Division Multiple Access (CDMA) cellular communications system.
2. Description of Related Art
In wideband spread spectrum cellular communications systems (often referred to as Wideband-CDMA or W-CDMA systems), power control symbols are transmitted on a regular basis in order to compensate for the effects of changing conditions that influence the channel, such as fading and shadowing. For example, a serving base station can transmit a power control command, which signals a served mobile station to increase or decrease its transmission power level. The mobile station then decides whether to increase or decrease its transmission power level in response to the power control command received.
However, for a mobile station operating in a soft handoff (SOHO) mode, it commences communications with a new base station before terminating communications with the old base station. Consequently, the mobile station""s transmission power level decisions have to be made based on the power control commands received from the two (or more) base stations that are serving the mobile station during the SOHO operation. Similarly, on the downlink, each base station can decide what transmission power level is needed for each served mobile station, based on the power control messages received from the served mobile stations. With this approach, the effects of changes to the channel conditions can be minimized.
When a mobile station is operating in a non-SOHO mode, the mobile station""s decision about whether to increase or decrease its transmission power level can be based on the direction provided by the power control bit or symbol received from its serving base station. As such, as specified in the technical standards involved (e.g., the ARIB standards for Wideband-CDMA, and the IS-95 Standard: ANSI J-STD-008, pp. 2-137), the amount that the mobile station is directed to increase or decrease its transmission power level is a constant value.
A similar power control approach is taken on the downlink for base stations operating in a non-SOHO mode. A serving base station""s decision about whether to increase or decrease its transmission power level is a hard decision, which is based directly on the detected power control bit or symbol transmitted from a specific served mobile station. However, in the proposed ETSI standard (for Europe), the amount that the serving base station will increase or decrease its transmission power level is variable, but not on a slot level. In other words, the network can select an xe2x80x9camountxe2x80x9d for the base station to use, but once that amount is selected, it remains constant until the next change (e.g., triggered from a higher protocol level).
On the other hand, when a mobile station is operating in a SOHO mode, the decision about whether to increase or decrease its transmission power level is based on the power control bits or symbols received from the two (or more) base stations involved in the SOHO procedure. In accordance with the IS-95 standard, the mobile station will increase its transmission power level only when all of the base stations involved in the SOHO procedure convey a power increase command. However, if just one of those base stations transmits a power decrease command during a SOHO procedure, the mobile station is required to decrease its transmission power level. In any event, the mobile station""s transmission power level is increased or decreased by a constant value.
Also, for W-CDMA systems, each base station involved with a mobile station in a SOHO procedure independently receives a power control bit or symbol from that mobile station. Therefore, each such base station can decide independently from the other base station(s) whether to increase or decrease its transmission power level for the mobile station involved, based on the power control bit or symbol received (and detected) from that mobile station.
A significant problem encountered when a base station or mobile station has to make a power control decision in a spread spectrum or CDMA cellular system is that it requires the base station or mobile station to make a so-called xe2x80x9chardxe2x80x9d decision (i.e., evokes a precise response) based solely on the proper detection of a received power control bit or symbol. However, it is always possible that a received power control bit or symbol will be erroneously detected. Consequently, the existing power control approaches that require such a hard decision (based solely on the proper detection of a power control symbol) are not optimal approaches.
Furthermore, when operating in a SOHO mode, a hard decision by a mobile station to increase its transmission power level in response to power increase commands from all base stations involved in the SOHO procedure is also not an optimal decision, because the probability that at least one power control symbol has not been received or has been erroneously detected increases with the number of base stations involved. Nevertheless, as described in detail below, the present invention successfully resolves the above-described problems.
In accordance with the present invention, a method for controlling the power transmission levels in a spread spectrum or CDMA cellular system is provided whereby the decision made to increase or decrease the transmission power level of a mobile station or base station, and the amount of any such increase or decrease made, is based on the weighted combination of a plurality of factors that indicate existing channel conditions. In a preferred embodiment of the present invention, the weight is set according to the reliability of each channel condition factor involved in making the power control decision.
An important technical advantage of the present invention is that an optimal power control decision can be made by a base station or mobile station in a spread spectrum or CDMA system based on all channel condition information available.
Another important technical advantage of the present invention is that it provides a better solution for existing power control problems in SOHO situations than provided by conventional approaches.
Still another important technical advantage of the present invention is that it provides a more efficient method for setting system power control parameters in spread spectrum or CDMA systems.
Yet another important technical advantage of the present invention is that it optimizes the utilization of available radio resources in a spread spectrum or CDMA system, such as transmission power, system capacity, etc.
Still another important technical advantage of the present invention is that it provides an optimal method for making power control decisions in a spread spectrum or CDMA system, which gives due consideration to such factors as perceived channel conditions and power control statistics.